1. Field of the Invention
The present invention relates to a charging control system and a charging control device for controlling the charging of a secondary battery from a solar battery and reducing the loss of power from the secondary battery when the charging path is interrupted.
2. Description of the Related Art
A known type of charging control system, controls the charging of a secondary battery from a solar battery by means of a switching element that prevents the secondary battery from discharging through the solar battery when not being charged by the solar battery.
The main elements of the charging control system are a solar battery, a secondary battery, and a charging path interconnecting the solar battery and secondary battery. The system also includes a first current path interconnecting the high side charging path and low side charging path through a pair of resistors, a second current path interconnecting the high side charging path and low side charging path through another pair of resistors, a comparator for comparing the voltage of the solar battery as divided by the first pair of resistors with the voltage of the solar battery as divided by the second pair of resistors, and a p-channel metal-oxide-semiconductor (PMOS) transistor inserted in high side charging path between first current path and the second current path to interrupt the charging path responsive to the output of the comparator. The secondary battery, part of the charging path, and first current path form a closed circuit.
This conventional system operates as follows. The comparator compares the voltage of the secondary battery as divided by the second pair of resistors on the first current path with the voltage of the solar battery as divided by the first pair of resistors on the second current path. If the difference between the voltage of the solar battery and voltage of the secondary battery exceeds a threshold value, the comparator outputs a low-level signal to turn on the PMOS transistor, thereby charging the secondary battery from the solar battery; if the difference is equal to or less than the threshold value, the comparator outputs a high-level signal to turn off the PMOS transistor, thereby interrupting the charging of the secondary battery from the solar battery.
A problem in this conventional charging control system is that even when the switching element is turned off to interrupt the charging of the secondary battery from the solar battery, current continues to flow from the anode of the secondary battery to the cathode of the secondary battery through the first current path, thereby discharging the secondary battery and wasting some of the power stored in the secondary battery.